I have a very old Honeywell programmable thermostat. I am going to replace it with a White-Rodgers UP500W Emerson Sensi WiFi Thermostat.

I just want to know if the WiFi unit will work without a C wire. This is what they said:

In certain situations, Sensi does require a common or "C" wire. On "heat only" or "cool only" systems, Sensi requires a "C" wire to power the Wi-Fi radio. Without a "C" wire installed, the Wi-Fi connection will drop off after about two hours to preserve the AA batteries. On combined heating and cooling systems, Sensi does not require a "C" wire because the thermostat is designed to "Power Share" between the two systems. For example, on a call for heat, Sensi will use a small amount of power from the cooling system to power the Wi-Fi radio. In "Power Sharing" mode, the AA batteries may last more than a year.

Since the one I have is for both heating and cooling, I think I should be fine. I just need someone to confirm that.

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  • 1
    Why not just install a C wire? It looks like you have an extra blue wire in the cable, why not just hook it up?
    – Tester101
    Commented Jun 17, 2015 at 1:07
  • If this answer isn't enough to help you install a C wire, just ask a new question being sure to include the make and model number of the heating and cooling equipment you're using.
    – Tester101
    Commented Jun 17, 2015 at 1:10
  • @Tester101 , I opened it up again, I do have 2 extra wires, one is blue, and one is brown. does it matter which one I use?
    – qinking126
    Commented Jun 17, 2015 at 1:22
  • 1
    It's more common to use the blue wire, but it really doesn't matter which one you use. You will have to connect whichever wire you choose to both the thermostat, and the HVAC system.
    – Tester101
    Commented Jun 17, 2015 at 2:26

2 Answers 2


This is my attempt to "demystify" the whole "C" wire thing for non-technical folks. This explanation intentionally ignores lots of details that are not relevant to a basic understanding of the concept.

An old-timey thermostat is mechanical in nature. You can think of it very much like 3 separate mechanical on/off switches. (the "switches" in this case are mechanically actuated by temperature using springs and tilting glass bulbs full of mercury and other old-timey mechanical mechanisms)

The circuit is actually VERY simple. The old-timey thermostat switches on and off the "juice" which comes in on the RED wire...separately on and off to each of the other 3 wires (often green for the fan, white for the heat, and yellow for the AC).

That's all the old-timey thermostat does. Hook or not hook the red wire to each of the other three as needed.

Modern thermostats control your HVAC system with the same connections. They do the same thing...hook the red wire up to the proper green/white/yellow to turn various parts of you HVAC on and off as needed.

Since the old-timey thermostats were mechanical in nature, they didn't need any juice of their own to operate. If you think of a modern thermostat as an old-timey thermostat plus a smart phone stuck to the wall right next to it...you need something to power the smartphone. (some "kind of modern" thermostats have batteries in them for this purpose...to power the "smarts")

The red wire brings juice into the thermostat...but there is no "ground wire". Our "smartphone stuck on the wall next to the old-timey thermostat" needs both a "hot wire" and a "ground" (or "common"). (again, this is simplified so as to not get lost in irrelevant details and confuse the non-technical readers this was written for)

The "C Wire" (often blue) is this additional "ground wire" needed in order to complete a circuit for the electronics (which old-timey thermostats didn't have). The "other end" of the C wire is hooked to the other side of the transformer that supplies juice to the R or red wire. It is the other wire needed by the electronics in the newer thermostats in order to power up their "smarts".

Hope this helps a few people understand what the C wire is about.


You need the c wire to power the wifi thermostat. Depending on your system you may have an extra wire not being used. Check with a multimeter to see if any other lines are feeding power, and not being switched. Ie in essence acting like a c wire. If you have another wire not being used connected it to the power of your furnace.

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